In charged-particle-beam (CPB) lithography using an electron-beam, an electron beam emitted by an electron gun is collimated with a condenser lens and then deflected by a deflector to irradiate a selected sub-field of a reticle. The selected sub-field defines a pattern to be projected and transferred to a corresponding location on a sensitized substrate. The substrate is typically a semiconductor wafer that has been coated with a resist sensitive to the electrons. The electron beam is shaped by a field stop before irradiating the reticle so that only the selected sub-field is irradiated. Because the edge of the field stop scatters or reflects a portion of the electron beam, electrons are also incident on unselected locations of the reticle. These electrons generally reach the sensitized substrate, reducing the contrast of the projected images and improperly exposing unselected regions of the sensitized substrate. To reduce scattering from the edge of the field stop, conventional apparatus use field stops that have edges tapered or thinned to resemble knife-edges.
In a conventional apparatus, a condenser lens is placed between the reticle and the electron gun and a projection lens is placed downstream of the reticle. The condenser lens and the projection lens are generally similar lenses. As a result, the distance between the condenser lens and the reticle is large, causing the image of the field stop projected on the reticle to be a magnified (enlarged) image.
Such an apparatus has several disadvantages. In systems in which the field stop is magnified onto the reticle, the current density of the electron beam is larger at the field stop than at the reticle. Because the edge of the field stop is tapered or thinned to reduce scattering, the combination of a high current density and a high beam-acceleration voltage heat the field stop edge sufficiently to melt it.
If a single condenser lens is used between the field stop and the reticle, then the image of the field stop on the reticle varies as the image of the field stop is directed to various reticle sub-fields. In some cases, the magnification of the image of the field stop on the reticle changes if the numerical aperture of the electron beam changes. In such cases, the size of the field stop must be changed whenever the magnification is changed so that the image of the field stop remains a constant size.
When sequentially irradiating sub-fields of the reticle, the electron beam is blocked ("blanked") by a blanking aperture as the beam is directed to the next sub-field. In a conventional apparatus, the irradiation of the sub-fields does not remain uniform as the electron beam is in a transition to a blanked condition, even if the irradiation by the unblanked beam is uniform. In addition, the irradiation of sub-fields that are not situated along the optical axis ("off-axis") is non-uniform because of distortion in the image of the field stop.